Accurate measurement of liquid flow in services such as oil field production, pipelines, injection of catalysts, catalyst inhibitors, co-catalysts, laminar flow, slurries (multi-phase liquid/solid suspensions), "pure" liquids, solutions, liquid/gas suspensions, pulsating systems and the like has long been a problem. Flowmeters in the past have largely depended upon knowing the density, viscosity, and composition of the liquids, and assumes that steady state flow conditions are being measured. Various flowmeters have always been subject to errors such as calibration shifts, orifice restrictions, bearing drag and blade wear of turbine meters when inserted into liquid flow for measurement. The problem is particularly acute with respect to flow of high viscosity liquids, or flow which contains contamination or quantities of solids, or flows in the transition or laminar regions, or flow streams which are in essence slurries. Liquids of this type are typically metered by an integral orifice meter at lower flow rates. Such meters are subject to partial plugging by the metered liquid and the solids contained therein, which partial plugging results in an "unknown flow" such that, one flow is reported, while a second unknown flow actually flows. This "unknown flow" is often discovered only by product deterioration or by lost product at varying times after the metered flow no longer accurately reports the actual flow. Often, such partial plugging spontaneously clears. However, as such clearing often occurs after steps are taken to correct or adjust the unknown flow, a period of end product variability results, resulting in large amounts of end product waste and a resultant unknown flow through the unplugged orifice.
In many processes, where possible, these problems are avoided by simply injecting an over supply of the required liquid. In other processes, an over supply results in a non-desired product and such a simple excess will not effectively overcome the lack of knowledge of liquid flow. In such situations, it has been common to use two or more meters in series, but it is difficult to reconcile differing readings and to be sure which, if either, reading is correct. Other situations, such as intercompany or interplant transfer of liquids through pipelines, often result in inaccurate charges and oversupply or supply deficiencies in the fluid transferred.
The "unknown flow" costs industry considerable sums of money in off-specification product, liquid oversupply and undersupply, or other forms of waste. It would be highly desirable to provide an apparatus and method for determining mass flow accurately, reproducibly and preferably in a self-checking, self-correcting flowmeter which can adjust flow to a present target level, and correct its own output signal to give the true flow.
Representative but non-exhaustive examples of prior art attempts to measure and control flow is represented by U.S. Pat. No. 3,001,397 which utilizes a two-reservoir system having a valve between reservoirs which measures the change in reservoir level when the valve is closed for a pre-determined time. U.S. Pat. No. 4,353,482 is an example of conventional wet and dry blend feeding and metering systems. U.S. Pat. No. 4,397,189 describes a method for measuring low levels of liquid flow rates but does not provide continuous flow measurement, does not measure mass, and diverts fluid flow for measurement readings rather than processing all fluids.
Some flowmeters utilize a coriolis effect, where the liquid flows through at least one tube and deflects the tube, where the degree of deflection over time is used to derive a mass flow reading. Such meters are subject to errors introduced by manufacturing, materials of construction, calibration, abrasive wear on the tube, and product coating.
Coriolis meters are normally made of stainless steel, but in some applications other materials must be used, such as tantalum, titanium, or the like, thus limiting accuracy. The apparatus of the present invention can be made from liquid-containing material of choice without loss of accuracy.
This invention is an improvement over our mass flowmeter apparatus described in U.S. Pat. No. 4,718,443. Our previous invention was made in response to a continuing need for accurate measurement of mass flow without knowing the density, composition or viscosity of the liquids measured. The invention described admirably filled those needs, but had one significant disadvantage: the requirement that the calibration tank vent while filling. Changes in column pressure were believed to adversely affect the instrument error.
It would be of great benefit to provide an apparatus capable of accurately measuring mass flow without the necessity of venting the calibration tank on the fill cycle.
It would be of great benefit to provide an apparatus and method for accurate measurement of flow as well as a means to correct the flow in the event of partial plugging of the meter, or a change in inherent meter performance, or a change in operating conditions, which in turn changes the fluid properties being measured.
It is an object of the present invention to provide an apparatus and a method for measuring liquid mass flow without venting the calibration tank on the fill cycle. Other objects will become apparent to those skilled in the art as the description proceeds.